Galaxy

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Galaxy

A galaxy is a large collection of stars, glowing nebulae (clouds), gas, and dust bound together by gravity. Many scientists now believe that a black hole, the remains of a massive star, lies at the center of many galaxies. Galaxies are as plentiful in the universe as grains of sand on a beach. The galaxy that contains our solar system is called the Milky Way. The Milky Way is part of a cluster of some 30 galaxies known as the Local Group, and the Local Group is part of a local supercluster that includes many clusters.

Although astronomers are not yet sure how galaxies formed and evolved, the process must have occurred quickly very early in the history of the universe. The age of the oldest galaxies appears to be about the same age as the universe, which is estimated to be 10 to 13 billion years old.

Words to Know

Barred spiral galaxy: A spiral galaxy in which the spiral arms start at the end of a central bar structure rather than the nucleus.

Black hole: The remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.

Dark matter: Unseen matter that has a gravitational effect on the motions of galaxies within clusters of galaxies.

Halo: A distribution of older stars and clusters of stars surrounding the nucleus of a spiral galaxy.

Irregular galaxy: A galaxy that does not fit into the shape categories of elliptical and spiral galaxies.

Nebulae: Bright or dark clouds, often composed of gases, hovering in the space between the stars.

Nucleus: The central core of a galaxy.

Radio waves: Electromagnetic radiation, or energy emitted in the form of waves or particles.

Spiral arms: The regions where stars are concentrated that spiral out from the center of a spiral galaxy.

Spiral galaxy: A galaxy in which spiral arms wind outward from the nucleus.

The shape of galaxies

Galaxies can be spiral, elliptical, or irregular in shape. The Milky Way and nearby Andromeda galaxy are both spiral shaped. They have a group of objects at the center (stars and possibly a black hole) surrounded by a halo of stars and an invisible cloud of dark matter. From this nucleus or center, arms spiral out like a pinwheel. The spiral shape is formed because the entire galaxy is rotating, with the stars at the outer edges forming the arms. Most spiral galaxies have just one arm wrapped around the nucleus, although some have two or even three arms.

Spiral galaxies are divided into two types: barred and unbarred. In barred spirals, a thick bar of stars crosses the center of the galaxy. Unbarred spirals have no such feature.

An elliptical galaxy contains mostly older stars, with very little dust or gas. It can be round or oval, flattened or spherical, and resembles the nucleus of a spiral galaxy without the arms. Astronomers do not yet know whether elliptical galaxies eventually form arms and become spirals, or if spiral galaxies lose their arms to become elliptical.

About one-quarter of all galaxies are irregular in shape and are much smaller than spiral galaxies. The irregular shape may be caused by the formation of new stars in these galaxies or by the pull of a neighboring galaxy's gravitational field. Two examples of an irregular galaxy are the Large and Small Magellanic Clouds, visible in the night sky from the Southern Hemisphere.

Some galaxies are variations of these types. There are the Seyfert galaxies (violent, fast-moving spirals); bright elliptical galaxies of super-giants that often consume other galaxies; ring galaxies that seem to have no nucleus; twisted starry ribbons formed when two galaxies collide; and others.

The Milky Way

The Milky Way is a barred spiral galaxy about 100,000 light-years across. Its disklike nucleus, which bulges to about 30,000 light-years thick, contains billions of old stars and maybe even a black hole. It has four spiral arms. Our solar system is located in the Orion arm, about 30,000 light-years from the center of the galaxy.

Just as Earth revolves around the Sun, the Sun revolves around the nucleus of the galaxy. Traveling at a speed of about 155 miles (250 kilometers) per second, the Sun completes one revolution around the galactic center in about 220 million years.

Active Galaxies

An active galaxy is one that emits far more energy than a normal galaxy. The Milky Way, like most galaxies, is relatively stable and quiet. Active galaxies, on the other hand, give off more than 100 times the energy of the Milky Way. Explosions at the nucleus of active galaxies spew huge jets of material hundreds of thousands of light-years into space. The energy is emitted as radio waves (electromagnetic radiation) rather than optical light. There are several varieties of active galaxies, including Seyfert galaxies and quasars.

Seyfert galaxies look like spiral galaxies with a hyperactive nucleus. The normal-looking spiral arms surround an abnormally bright nucleus. Quasars are the most interesting of active galaxies. A quasar can emit more energy in one second than our sun has in its entire lifetime. Quasars, which look like stars, are the most distant and energetic objects in the universe known so far. Most astronomers consider a quasar to be the very active nucleus of a distant galaxy in the early stages of evolution. The light from a quasar has been traveling toward Earth for billions of years, perhaps from the very beginning of the universe.

In ancient times, people looked into space and saw a glowing band of light. They thought it resembled a river of milk and called it the Milky Way. In the late 1500s, Italian physicist and astronomer Galileo Galilei (1564–1642) first examined the Milky Way through a telescope and saw that the glowing band was made up of countless stars. As early as 1755, German philosopher Immanuel Kant (1724–1804) suggested that the Milky Way was a lens-shaped group of stars, and that many other such groups existed in the universe.

Over the years, astronomers learned more about the shape of the Milky Way, but they continued to place our solar system at the center. In 1918, American astronomer Harlow Shapley (1885–1972) studied the distribution of star clusters and determined that our solar system was not at the center, but on the fringes of the galaxy.

Hubble and the expanding universe

In 1924, American astronomer Edwin Hubble (1889–1953) first proved the existence of other galaxies. Using a very powerful 100-inch (254-centimeter) telescope at Mount Wilson Observatory in California, he discovered that a group of stars long thought to be part of the Milky Way was actually a separate galaxy, now known as the Andromeda galaxy. Modern estimates place Andromeda 2.2 million light-years away from the Milky Way. Hubble also discovered many other spiral-shaped galaxies. In 1927, Dutch astronomer Jan Oort (1900–1992) showed that galaxies rotate about their center.

Beyond these important discoveries, Hubble found that more distant galaxies are moving away from us at a faster rate. From this observation, known as Hubble's Law, he deduced that the universe is expanding, a fundamental fact about the nature of the universe.

In early 1996, the Hubble Space Telescope sent back photographs of 1,500 very distant galaxies in the process of forming, indicating that the number of galaxies in the universe is far greater than previously thought. Based on this and other discoveries in the late 1990s, astronomers have estimated the number of galaxies to be 50 billion.

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galaxy

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

galaxy, large aggregation of stars, gas, and dust, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926–29) by Edwin P. Hubble that indicated the great distances at which this and other galaxies are located. Previously, the galaxies had been classified with the luminous gas clouds, or bright nebulae, within the Milky Way. The sun and its solar system, as well as the visible stars, are all in the Milky Way galaxy. Harlow Shapley encouraged the exclusive use of the term
"galaxies."
Billions of galaxies are within the optical range of the largest telescopes; in 1996 analysis of photographs taken from the Hubble Space Telescope increased the estimated number of galaxies from 10 billion to 50 billion. Research with the Hubble Space Telescrope has also indicated that supermassive black holes are found at the center of large galaxy. A galaxy is held together by the gravitational attraction between its constituent parts (see gravitation), while its rotational motion prevents it from collapsing on itself. Just as gravitation binds individual stars into galaxies, it also acts to hold clusters of galaxies together. Many large galaxies have smaller galaxies, called satellite galaxies, in close proximity. The galaxies nearest the Milky Way form a cluster called the Local Group. The Local Group includes the Andromeda Galaxy, which is a spiral galaxy similar in size and composition to the Milky Way, and the Magellanic Clouds, which are satellite galaxies of the Milky Way. The vast majority of observed galaxies are classified as either spiral or elliptical (football-shaped), with a small minority, e.g., the Magellanic Clouds, classified as irregular according to a scheme originated by Hubble. Although estimates of the age of the universe have varied (see Hubble's law), current estimates place its age at around 13.75 billion years. The earliest galaxies imaged by modern astronomers include several that were formed less than 600 million years after that, and one galaxy has been discovered that was formed some 380 million years after the big bang.

A typical spiral galaxy is shaped like a flat disk, about 100,000 light-years in diameter, with a central bulge, or nucleus, containing old stars; winding through the disk are the characteristic spiral arms of dust, gas, and young stars (see stellar populations). This type of galaxy is further classified as being either a normal or a barred spiral. In the normal spiral, the arms, at least two in number, join smoothly with the nucleus; in the barred spiral, such as the Milky Way, the arms project from a bank of stars that runs through the nucleus. The elliptical galaxies, lacking spiral arms entirely and containing little or no gas and dust, resemble the nuclei of spiral galaxies. Their shapes vary from nearly spherical to highly flattened ellipsoids. Elliptical galaxies have a much greater variation in size, mass, and luminosity than do spiral galaxies; their sizes range from the largest known galaxies of all, with luminosities about 10 times that of the Andromeda Galaxy, to the small dwarf ellipticals, which can contain as few as a million stars. Irregular galaxies appear structureless and without any nucleus or rotational symmetry; their light comes mostly from young stars.

Spiral galaxies contain a larger number of bluer, younger stars, while elliptical galaxies contain a larger number of redder, older stars. This has led astronomers to believe that stars initially cluster into spiral galaxies and that over time structural changes transform them into elliptical galaxies. Some researchers speculate that the transformation occurs because of gravitational forces exerted by galaxies as they slowly pass each other. Computer simulations suggest another alternative, called
"galactic harassment,"
in which galaxies interact although they remain far apart and pass each other at high speeds. The most widely accepted alternative suggests that the transformation is caused by collisions of galaxies and gravitational tidal interactions between them as they travel through space, causing them to grow and evolve. Several dwarf galaxies are currently colliding with the Milky Way; others are on course to do so over the next 2 to 3 billion years. The collisions are not cataclysmic because galaxies—even though they may contain many billions of stars—are mostly
"empty"
space and the probability of two stars meeting is very small. However, the
"empty"
space is not really empty, it is full of gas and dust which can interact when the galaxies collide. There is also friction between the gas and dust in the colliding galaxies, causing shock waves that can trigger some star formation in the galaxies. These processes can radically affect the colliding galaxies, e.g., two spiral galaxies can merge to form an elliptical galaxy.

Many galaxies radiate a large fraction of their energy in forms other than visible light. With the development of radio astronomy, many radio galaxies were discovered. Other galaxies radiate strongly in the infrared, ultraviolet, or X-ray parts of the spectrum.

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galaxy

galaxy Huge, gravitationally bound, assemblage of stars, dust, gas, and dark matter. Current theories suggest all galaxies were formed from immense clouds of gas soon after the Big Bang. There are three main types, as classified by Edwin Hubble in 1925. Elliptical galaxies(E) are round or elliptical systems, showing a gradual decrease in brightness from the centre outwards. Spiral galaxies(S) are flattened, disc-shaped systems in which young stars, dust and gas are concentrated in spiral arms coiling out from a central bulge, the nucleus. Barred spiral galaxies(SB) have a bright, central bar from which the spiral arms emerge. In addition to these three main classes are transitional lenticular galaxies, systems with a disc and nucleus but with no apparent spiral arms. Irregular galaxies, such as the Magellanic Clouds, are systems with no symmetry. Galaxies can exist singly or in clusters. Interactions between gas-rich galaxies can produce bursts of star formation and may act as a trigger for generating active galactic nuclei such as quasars. About one galaxy in a million is a radio galaxy, emitting strong electromagnetic radiation. A Seyfert galaxy has a bright, compact nucleus and is a strong emitter of infrared waves. A general property of galaxies seems to be the presence of much more mass than can be accounted for by all the material that can currently be detected. This ‘dark matter’ makes up 90% of the mass in typical galaxies. It seems that supermassive black holes lie at the centre of most galaxies, including our own. Our own Galaxy, the Milky Way, is spiral and c.100,000 light years in diameter. Earth is 26,500 light years from the centre. The Solar System lies at the edge of one of the spiral arms, c.30,000 light years from the centre.

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galaxy

gal·ax·y
/ ˈgaləksē/
•
n.
(pl. -ax·ies)
a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. ∎
(the Galaxy)
the galaxy of which the solar system is a part; the Milky Way.
∎ fig.
a large or impressive group of people or things:
a galaxy of boundless young talent.ORIGIN:late Middle English (originally referring to the Milky Way): via Old French from medieval Latin galaxia, from Greek galaxias (kuklos) ‘milky (vault),’ from gala, galakt- ‘milk.’

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galaxy

galaxy a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction; the Galaxy, the galaxy of which the solar system is a part, the Milky Way; in figurative usage, a large or impressive group of people.

The name is recorded from Middle English, referring to the Milky Way, and comes via Old French and medieval Latin from Greek galaxias (kuklos) ‘milky (vault)’, from gala, galakt- ‘milk’.

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